Generally, a rigid polyurethane foam is obtained by reacting a polyol component and an isocyanate component under existences of reaction catalysts, a foam blowing agent and a foam stabilizer. The rigid polyurethane foam is a heat insulating material having the highest heat insulating property among organic and inorganic thermal insulators, and is used a lot in a refrigerator, a freezing container, a low temperature storage, etc. which require a higher heat insulating property. That is because the polyurethane foam consists of closed cells to have a higher heat insulating property, and a low density foam can be prepared by controlling a used amount and kind of the foam blowing agent. However, a polyurethane foam having lower density and thermal conductivity than that usually used, and having a good plasticity is highly required.
Generally, the rigid polyurethane foam is needed to be hardened in a foaming jig (mold) under a predetermined temperature range (40° C.˜55° C.) for a predetermined time after a raw solution is injected. In addition, a difference in post-expansion rate when demolding in the jig is generated according to the hardening time. The post-expansion rate after foam blowing of the polyurethane foam is directly related to size and changing amount of the refrigerator products, and basically, the amount of post-expansion after the foam blowing is increased in proportion to the size and thickness.
However, as the produced amount of the refrigerator is increased, as structure and size of the product become complex and large, and as quality control of the products becomes rigid, it is required that the expansion rate of the foam and the hardening time are reduced. However, it can not be realized by the conventional polyurethane foam, and the number of jig is increased, and thereby, the producing cost is increased.